1. Field of the Invention
The invention relates to an electron accelerator comprising a target exposed to an electron beam for the production of x-ray deceleration radiation and also comprising a massive cone-shaped compensating member which is centrally arranged in the x-ray cone.
2. Description of the Prior Art
In the case of electron accelerators x-ray deceleration radiation is produced due to a deceleration of the electrons in a so-called target. It is known in the art to balance or compensate the dosage in a given space-angle range of the x-rays leaving the target by placing a compensating member into the portion of the x-ray cone of interest. This compensating member has a conical design. Its contour path is adpated to the path of the radiation intensity at the place of use. Since the dosage decreases very markedly with the distance from the center beam behind the target, the sides of the compensating member are correspondingly steep and the tip of the compensating member must be positioned very precisely with respect to the center beam.
In the case of a properly employed compensating member, the intensity distribution as indicated by broken line 36 in FIG. 1 and which would be assumed by the beam cone leaving the target at the location plane of a patient undergoing treatment would be changed flattened continuous line 37. The compensating member absorbs the overly intense radiation in the center as compared with the margins of a given beam cone. The portion of beam cone having the intensity distribution represented by the horizontal portion of curve 37 in FIG. 1 can be used for radiation purposes. It is a disadvantage, therefore, that, even for exact positioning, misadjustments of dose compensation can occur due to minor fluctuations of the direction of the electron beam leaving the accelerator.
In order to decrease the difficulties encountered by adjustment of the compensating member, it has been previously suggested to place the compensating member further away from the target in a range of the beam cone where the latter has already clearly widened. This, however, has the disadvantage that the compensating member is then arranged closer to the patient. Also, a portion of the beam which scatters unavoidably in the compensating member along with its source, has also been placed closer to the patient. Due to the square distance law, this causes an increased radiation stress for the patient even with a relatively low-energy beam component. Furthermore, due to the increase of the spacing between the compensating member and the target, the entire beam defining system becomes larger and heavier.